In the Pali Day-by-Day answers, on http://www.tipitaka.net, the answer is givenas: "Boys, [together] with friends, see the king."

But I had translated it as: "Boys see the king with [his] friends."

Would someone please explain why my answer is not correct? How does one know that the word"mittehi" goes with "kumaaraa" and not with "bhuupaala.m"?

When using saha or saddhiṃ, normal Pali word order requires the things/persons accompanying to be stated immediately after the things/persons accompanied. To express your translation the word order would have been: